This invention relates generally to members useful in fusing apparatus for heat-fixing a heat-softenable toner material to a substrate. More particularly, the invention relates to a heater member useful for transferring heat to a fuser member in said fusing apparatus, wherein the heater member is externally heated and has an overcoat layer thereon comprised of a cured fluorocarbon thermoplastic copolymer composition described hereinafter.
Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner particles are deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield a final desired toner image.
When heat-softenable toners, comprising for example thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it, and then allowing or causing the toner to cool.
One such fusing method comprises passing the toner-bearing receiver sheet through a nip formed by a pair of opposing members, typically in the form of cylindrical rollers, wherein at least one of the members (usually referred to as a fuser member) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other member (usually referred to as a pressure member) serves to press the receiver sheet into contact with the fuser member. In some other fusing methods, the configuration is varied and the xe2x80x9cfuser memberxe2x80x9d or xe2x80x9cpressure memberxe2x80x9d can take the form of a flat plate or belt. The description herein, while directed to a generally cylindrical fuser roller in combination with a generally cylindrical pressure roller, should not be construed as limited to such a roller configuration.
The fuser member typically comprises a rigid core covered with a resilient material, which can be referred to as a base cushion. The resilient base cushion and the amount of pressure exerted by the pressure member serve to establish an area of contact for the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip formed by contact of the fuser member with the pressure member. The size of this area of contact helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness (often referred to as xe2x80x9cstorage modulusxe2x80x9d) and stability thereof, of the base cushion are important factors in establishing and maintaining the desired area of contact for fusing.
In some prior fusing systems, it has been advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a subtle xe2x80x9cbow tiexe2x80x9d or xe2x80x9chourglassxe2x80x9d shape. This causes the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as xe2x80x9ccreepxe2x80x9d) of the base cushion of the fuser member is a contributor to this problem.
While some fuser members are internally heated by placing a quartz lamp or other type of heat source internally within the fuser core, fuser members can also be externally heated by use of one or more external heater members, i.e., rollers, belts, plates or the like, placed in an opposed, contacting relationship with the fuser member. External heater members for fuser members can themselves be internally heated by use of a quartz lamp or other heat source. Apparatus for externally heating a heater member by a radiant heat source are disclosed in copending U.S. patent application Ser. Nos. 09/500,826 and 09/501,459 filed on Feb. 10, 2000, the teachings of which are incorporated herein by reference.
Heater members which are internally heated and used commercially have either an anodized surface or a very thin fluoropolymer resin, i.e., Teflon(copyright) fluorocarbon available from E.I. DuPont deNemours and Co. of Wilmington, Del., coating thereon, both of which have very low thermal resistance due to the relative thinness of such coatings. However, such heater members, when used in an opposed and contacting relationship adjacent to a fuser member, are not resilient or conformable, and therefore, do not allow for a relatively large area of contact (referred to as a xe2x80x9cnip widthxe2x80x9d hereinafter) with the fuser member when a nip is formed by contact of the heater member with the fuser member. Further, such coatings also have little or no ability to store heat. This arrangement results in inefficient heat transfer and undesirable heat loss.
A greater area of contact between the heater member and fuser member would allow for greater and more efficient heat transfer to the surface of the fuser member. To achieve a longer nip width, a conformable elastomer layer could be applied to the heater member. For internally heated heater members, however, a disadvantage with the use of such an elastomer layer is that it could create a time delay for heat energy to transfer to the surface of the heater member due to an increase in thermal resistance. A time delay would increase thermal response time when altering the fuser member surface temperature for any process reason. This increase in thermal response time could preclude the use of image gloss control by making changes in the fuser member temperature, or gloss and fusion tuning for various receiver types. Various receiver types, such as papers or films, have different thermal properties that can affect gloss and fusion quality. Having the ability to change the fuser member surface temperature rapidly within the time between consecutive receiver sheets allows fusion and gloss to be tuned to receivers within a document run that are of different types without reducing the productivity of the entire electrophotographic system. The foregoing ability to control gloss is particularly important for color electrophotographic systems.
U.S. patent application Ser. No. 09/501,459 previously mentioned herein, discloses a heater member which is externally heated and comprised of a core; a fluoroelastomer foam layer, such as Viton(copyright) fluoroelastomer available from DuPont, overlying the core; and an outer cured poly(perfluoromethylvinylether) layer thereover, such as a Kalreze(copyright) polymer also available from DuPont. While this externally heated heater roller is an improvement over prior commercially used internally heated heater rollers, the fluoroelastomer foam layer disclosed therein may not have sufficient mechanical strength in some apparatus designs to withstand stress imposed by what is known in the art as xe2x80x9cvelocity overdrivexe2x80x9d. As a result, the polymeric layers placed over the core could delaminate therefrom, thereby causing premature failure. Further, the poly(perfluoromethylvinylether) material is difficult to dissolve in commonly used solvents, thereby making it difficult to solvent coat onto the foam base cushion overlying the core. As a result, a sleeve of the material must generally be extruded and thereafter bonded to the foam base cushion, or molded and thermally bonded to the foam base cushion at high temperatures. These methods are generally more difficult to perform than solvent coating methods.
As can be seen, there is a need for a heater member, capable of being externally heated by a radiant heat source, which has a nip width, i.e., contact area, which can be set and/or varied so as to maximize and/or optimize heat transfer to the surface of an associated fuser member. It would also be desirable for the heater member to have an outer polymeric layer thereon in contact with the fuser member which is not only thermally stable, but also mechanically stable and more easily formed than other methods known to the art.
It is an object of the present invention to provide a heater member, capable of being externally heated by a heat source, which overcomes the limitations and disadvantages described hereinabove. It is a further object of the present invention to provide a heater member which is particularly suitable for external heating by a radiant source, and for use, for example, in an axially unsupported configuration in a fusing apparatus. According to the present invention, direct heating of the heater member surface allows for rapid changes in fuser and heater member surface temperatures, so as to alter the fusing process and provide gloss and/or image quality control between consecutive jobs and/or receiver sheets.
A method to accomplish the foregoing objectives according to the present invention employs an externally heated external heater member to impart thermal energy to a fuser member through conduction, i.e., by direct contact. The heater member has a conformable base cushion layer to increase nip width, i.e., contact width, and heating time, with the fuser member. The heater member is heated by an external radiant heat source, which imparts heat energy directly to the heater member surface and not solely by indirect heating means through the heater member core and overlying elastomer layer.
Thus, in one aspect, the present invention relates to a heater member for transferring heat to a fusing member used to heat fix a toner image to a substrate. The heater member comprises:
a core;
a conformable base cushion layer overlying the core; and
an outer layer overlying the base cushion layer,
wherein the outer layer comprises a cured fluorocarbon thermoplastic random copolymer.
In embodiments, the invention relates to a heater member for transferring heat to a fusing member used to heat fix a toner image to a substrate. The heater member comprises:
a core;
a conformable base cushion layer overlying the core; and
an outer layer overlying the base cushion layer,
where the outer layer comprises a cured fluorocarbon thermoplastic random copolymer having subunits of:
xe2x80x94(CH2CF2)xxe2x80x94, xe2x80x94(CF2CF(CF3))yxe2x80x94, and xe2x80x94(CF2CF2)zxe2x80x94,
wherein:
x is from 1 to 50 or 60 to 80 mole percent,
y is from 10 to 90 mole percent,
z is from 10 to 90 mole percent, and
x+y+z equals 100 mole percent.
The present invention provides an ability to change the fuser member surface-temperature during operation, thereby allowing for gloss and/or image quality control. It also provides better thermal droop management for the overall fusing system, so that it is not necessary to artificially increase and decrease the fusing member surface temperature to increase the stored energy within the fuser member, while trying to maintain a desired fusing temperature-control set-point.
The external radiant heat feature, particularly in combination with a preferred, relatively low thermal conductivity (i.e., thermally insulating) conformable base cushion layer as described hereinafter, can allow internal components within the heater member to remain cooler in comparison to an internally heated heater member system, which could either increase component life or reduce component cost if the component life requirement otherwise remains the same.
Another advantage of the present invention is that the cured fluorocarbon thermoplastic random copolymer materials employed allow for a relatively large temperature gradient to be formed between the surfaces of the fuser member and heater member, so as to increase available heat time or dwell.
Another advantage is that use of a preferred poly(organosiloxane) base cushion layer as described hereinafter allows for mechanical stability and also sufficient compression characteristics so that the resulting heater member has a conformable outer surface which can be adapted to form contact, i.e., pressure, nips of increased width and, therefore, greater surface area for heat transfer, with the associated benefits and advantages as previously described. A greater nip width allows more nip time and thereby enables high volume (or high speed) heating of the fuser member surface without undesirable thermal droop. The preferred silicone base cushion also generally allows for a pressure nip with significantly less velocity overdrive, which reduces relative motion in the nip, therefore reducing fuser member surface wear.